Internal combustion engines produce energy by combustion of a fuel with (usually) air in a combustion chamber. The combustion process in internal combustion engines produces power to move the vehicle, usually converting the linear motion within the combustion chamber to rotation, but also produces heat.
The combustion products—uncombusted fuel, unused oxygen, and byproducts, in the form of (often) hot exhaust gases—are expelled through an exhaust system taking the combustion products away from the engine. Exhaust gas heat recovery is designed to remove heat from the exhaust gas of engines and transfer it elsewhere, such as to a water circuit. The interior of the car may be warmed using exhaust heat.